1. Field of the Invention
The present invention relates to a video input processor for imaging a subject and suppressing noise components of produced imaging signals by signal processing. The invention also relates to an imaging signal-processing circuit and to a method of reducing noises in the imaging signals.
2. Description of the Related Art
When a subject is imaged by a camera device designed to image a motion picture sequence or still images, for example, with enhanced ISO sensitivity, the automatic exposure function is activated. This shortens the exposure time of the sensor (imaging device) and increases the gain for the output signal from the sensor. Therefore, the effects of noises produced by the sensor itself and by ICs located around it become more conspicuous. As a result, the final image becomes a noisy image with poor signal-to-noise ratio.
Noises are produced by factors associated with the sensor itself and by various other ambient factors such as the operating frequency of an IC located around the sensor. Furthermore, the image is affected by various noise patterns of from low to high frequencies due to variations in characteristics of the signal source (sensor) itself and timewise variations of the signal source.
A known technique for performing signal processing inside a camera device to reduce or suppress noise uses a circuit employing a filter having a large number of taps for noise reduction. The signal-to-noise ratio of the displayed image is improved by processing performed by this circuit.
However, if processing is performed by an ordinary low-pass filter (such as a Gaussian filter) having a large number of taps, edge information will be diffused concomitantly. In the output image, the steepness of each edge drops and the apparent resolution deteriorates. To solve this problem, a noise reduction technique which improves the S/N of the input image by performing filtering processing while maintaining the resolution may be necessary during processing of camera signals.
One known approach for noise reduction uses an imaging device equipped with given color filters such as red (R), green (G), and blue (B) filters. Light from a subject is passed through an IR (infrared radiation) cut filter for cutting invisible radiation components, especially in the near-infrared region. The radiation transmitted through the IR cut filter is received by the imaging device having the color filters. An image having high color reproducibility is outputted from the imaging device. In addition, the same subject is imaged, and the resulting image is not transmitted through the IR cut filter to obtain the image with a larger amount of information. Edge information is detected from the image whose amount of information is maintained by not allowing the image to pass through the IR cut filter (see, for example, JP-A-2006-180269 (patent reference 1)).
In the above-cited patent reference 1, the image for deriving the edge information is referred to as invisible light image (in the embodiment, referred to as the infrared light image). This image holds image information containing a wide range of frequency components from low to high frequencies, the image information being obtained before the image is passed through the IR cut filter.
In the above-cited patent reference 1, the invisible light image (infrared light image) is passed through a high-pass filter to extract high-frequency components. Meanwhile, the visible light image, i.e., the image which has high color reproducibility and which is captured after being passed through the IR cut filter, is adjusted in terms of gain. Then, the image is passed through a low-pass filter, removing noises. However, when the image is passed through the low-pass filter, resolution information is diffused, deteriorating the steepness of each edge. Therefore, in the technique of the above-cited patent reference 1, the visible light image passed through the low-pass filter and the infrared light image passed through the high-pass filter are combined, the infrared light image holding edge information. Consequently, noise reduction and prevention of diffusion of edge information (deterioration of the steepness of each edge) are achieved simultaneously.
With respect to the IR cut filter, a function of selectively transmitting and blocking infrared light components can be imparted to a single color filter layer of an imaging device.
For example, JP-A-2005-006066 (patent reference 2) discloses a color filter array whose pixel units are each made up of transmission filters of the three primary colors of red (R), green (G), and blue (B) and an infrared pass filter which has sensitivity to the infrared region and which transmits infrared light. Patent reference 2 states that the infrared filter can be a white (W) filter.
On the other hand, with respect to gain adjustment, color correction, and noise reduction performed during white balance adjustment, processing of imaging signals having infrared light components is known (see JP-A-2001-078204 and JP-A-2005-303704 (patent references 3 and 4)).